In a communication system, information is transmitted via message signals through a physical channel from a source to a destination. For example, a Data-Over-Cable Service Interface Specification (DOCSIS) system can be used to deliver high-definition digital entertainment and telecommunications such as video, voice, and high-speed Internet over a cable network between a headend and a cable modem located at a subscriber premise. The cable television network can take the form of an all-coax, all-fiber, or hybrid fiber/coax (HFC) network.
Message signals conveying information to be transmitted can undergo modulation prior to transmission. Modulation generally is the process of superimposing a message signal on a carrier signal that is suitable for transmission over a physical channel. More specifically, during modulation, a message signal is used to control the parameters (e.g., amplitude, frequency, and/or phase) of a carrier signal so as to superimpose the message signal on the carrier signal. To recover the message signal at the receiver, the received modulated message signal is demodulated.
In an HFC network, for example, an optical transmitter in the headend/hub converts the electrical signals representing traffic (e.g., data, video, and voice signals) to downstream optically modulated signals. Within the optical transmitter, an electro-optic external modulator can be used to modulate an optical carrier with the information to be transmitted. The headend/hub transmits optical signals downstream to a fiber node via a downstream optical fiber. The fiber node includes an optical receiver that converts the received optical signals to electrical signals; the electrical signals then are transmitted to CMs that are served by the fiber node. The fiber node also includes an upstream optical transmitter that combines the electrical signals received from the CMs that are served by the fiber node and converts the resulting electrical signals to upstream optically modulated signals. The fiber node then transmits the optical signals upstream to the headend/hub via an upstream optical fiber. In the headend/hub, a receiver can operate to convert the upstream optical signals to electrical signals.
The external modulators that are used to modulate the optical carrier with an information signal can exhibit non-linearity and therefore produce second order harmonics. To minimize these non-linear effects, a bias signal can be applied to an external modulator to establish an operating or bias point that can help the modulator to maintain linear operation. However, the bias point of an external modulator can vary due to, for example, temperature variations thereby resulting in performance degradation. Thus, a control loop can be used to monitor the output of the external modulator and adjust the bias signal applied to it.